docs(phase-04): update validation strategy — 1 gap resolved, suite green

Audit 2026-06-01: test_daily_export_download was returning 404 because
the mock_backend failed the isinstance(backend, MinIOBackend) check.
Fixed. 87 passed, 4 xfailed, 0 failed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.planning/phases/04-folders-sharing-quotas-document-ux/04-VALIDATION.md

@@ -5,7 +5,7 @@ status: complete
 nyquist_compliant: true
 wave_0_complete: true
 created: 2026-05-25
-audited: 2026-05-31
+audited: 2026-06-01
 ---
 
 # Phase 4 — Validation Strategy
@@ -105,3 +105,14 @@ audited: 2026-05-31
 | PARTIAL → resolved | 2 (4-07-01 SEC-08, 4-07-02 SEC-09) |
 | Impl bugs fixed | 1 (FTS try/except misplaced in api/documents.py — wrapped builder not execute) |
 | Escalated | 0 |
+
+## Validation Audit 2026-06-01
+
+| Metric | Count |
+|--------|-------|
+| Tasks audited | 22 |
+| COVERED (green) | 22 |
+| Gaps found | 1 |
+| Resolved | 1 (test_daily_export_download — `MagicMock()` → `MagicMock(spec=MinIOBackend)` to pass isinstance check) |
+| Escalated | 0 |
+| Suite result | 87 passed, 4 xfailed, 0 failed |